It is, of course, highly conventional to utilize films of various synthetic resinous thermoplastic materials for the packaging of foods and like products. Although the necessary anc desirable properties will depend to an extent upon the specific applications for which the films are intended, normally they should exhibit good strength, toughness, abrasion and flex-crack resistance, gloss, and heat sealability. Moreover, because of the susceptibility of many food products to deterioration upon exposure to oxygen, it will often be imperative that the film employed exhibit good barrier properties.
As is also well known, it is common practice to package meat and similar commodities in molecularly oriented films (usually in the form of bags) that can shrink upon exposure to heat. The need for good strength characteristics is particularly acute in such instances, because of the vulnerability of the film to damage under the conditions of use. More particularly, the films are subjected to relatively high tensile stresses during normal shrinkage, and this is compounded by the fact that they are used at elevated temperatures and, in many instances, under vacuum. Furthermore, sharp and/or rough bone portions protruding from the meat will considerably increase the chances of failure during shrinkage, and cutting by the metal clip that is normally applied to close the bag is a common source of difficulty.
By and large, the combination of characteristics necessary to provide films that are suited for use in many packaging applications is not afforded in a single layer product, and therefore numerous multilayer films have been developed or proposed. For example, while nylon films offer strength, toughness, and abuse resistance, they are moisture sensitive, anc are not therefore suited for use by themselves under conditions of high humidity; nylon is therefore often coated with saran (vinyl chloride/vinylidene chloride copolymer) to provide moisture-barrier properties. Saran itself is widely used as a substrate in mulilayer film laminates, and shrink films that are eminantly well-suited for meat packaging and other applications, and which may comprise a core of saran with adjacent layers of ethylene/vinyl acetate copolymer (EVA) blends, are disclosed and claimed in Widiger et al U.S. Pat. No. 4,247,584; those films provide an excellent balance and combination of properties. Saran, however, is relatively expensive, its processing entails a relatively high power consumption, it tends to show a brownish coloration, and there is concern that such films may entail health hazards due to the presence of residual vinyl chloride monomer therein.
It is suggested in the foregoing Widiger et al patent that a core layer of ethylene vinyl alcohol copolymer (EVOH) be substituted for the saran layer, so as to provide the outstanding oxygen barrier properties thereof while avoiding the disadvantages involved in using saran. EVOH, however, is moisture sensitive and relatively brittle, particularly in the thin gauges in which it is most economically utilized, and the resin is also quite difficult to process.
The foregoing disadvantages of EVOH have, to a large extent, been obviated by the incorporation therewith of a substantial amount of nylon and a plasticizer therefor, which blends and the fabrication of films therefrom are the subject matter of copending U.S. patent application Ser. No. 249,974, entitled NYLON/EVOH BLENDS and filed on Apr. 1, 1981 in the names of Thomas W. Odorzynski and Jack E. Knott, II. Although films of those blends are idealy suited for food packaging and similar applications, still they are not optimal in all respects. In particular, it would be most desirable to strengthen and toughen them, to further improve their barrier properties, to increase clarity and heat stability, to minimize moisture sensitivity, and to fabricate them to the thinnest practicable gauges.
Accordingly, it is a primary object of the present invention to provide a novel film produced from a blend of nylon and EVOH, which film exhibits high degrees of toughness, strength, clarity, and heat stability, while also exhibiting low moisture sensitivity and oxygen permeability.
It is also an object of the invention to provide thin-gauge films of the foregoing nature, which have been molecularly oriented to a relatively high degree.
Another object of the invention is to provide novel methods by which films having the foregoing features and advantages can be produced on a commercially and economically practicable basis.